Defects in the cholinergic system have been suggested to underlie cognitive impairments associated with normal aging and Alzheimer""s disease (Bartus et al., Science 217:408-417 (1982); Fisher et al., Neurobiol. Aging 13:9-23 (1992)). Much research has focused on the development of cholinomemetic replacement therapy as a potential treatment of these impairments. Among them, cholinesterase inhibitors, such as physostigmine (Phy) and tetrahydroaminoacridine (ThA) have been investigated for memory-enhancing effects in both animals (Rupniak et al., Neurobiol. Aging 11:09-613; 1990); Murray et al., Psychopharmacology 105:134-136(1991) and human patients (Mohs et al., J. Am. Geriatr. Soc. 3:749-757 (1985); Summers et al., N. Engl. J. Med. 315:1241-1245(1986)).
Other agents have been proposed as selective inhibitors of acetylcholinesterase (AChE). Thus heptyl-physostigmine (Heptyl-Phy) was described as having greater lipophilicity, longer inhibitory action on cholinesterase and more persistent increases in acetylcholine in brain with less toxicity than the parent compound (Brufani et al., Pharmacol. Biochem. Behav. 26:625-629 (1987)). There is concern, however, as to whether the therapeutic window of heptyl-Phy is wide enough for clinical use. Phenserine ((xe2x88x92)-N-phenylcarbamoyl eseroline) has been identified as a superior, selective AChE inhibitor and thus suited as an agent for the therapy for cognitive impairments associated with aging and Alzheimer""s disease. (U.S. Pat. No. 5,409,948, issued Apr. 25, 1995).
In U.S. Pat. No. 5,171,750 issued Dec. 15, 1992, a series of substituted phenserines are disclosed which are indicated to be either selective inhibitors of AChE or butyrylcholinesterase (BChE). The cumylcarbamate (4xe2x80x2-isopropylphenylcarbamate) derivative of (xe2x88x92)-physovenol was noted to have a reverse enzyme specificity, i.e., it inhibited BChE selectively over AChE. The patent indicates that the compounds of the invention are useful xe2x80x9cfor treating cholinergic disorders such as glaucoma, Myasthenia Gravis, Alzheimer""s disease and as an antidote against poisoning with organo phosphates.xe2x80x9d There is no indication as to which type of inhibitor would be used to treat the specified disorders, however, there is a further disclosure to the effect that AChE, which is found in red blood cells, in the brain and in nerve tissues, seems to be more specific an enzyme known to hydrolyze acetylcholine (ACh) in vivo than does BChE which is found in serum, pancreas and the liver. The marked cholinergic loss in AD is accompanied by dramatic reductions in the enzymes cholineacetyl transferase, involved in the synthsis of the cholinergic neurotransmitter acetylcholine, Ach, and of AChE, that ends the action of Ach (Perry, et al. Brit. Med. J., 2 6150: 1457-1459, 1978; Whitehouse, et al. Science 215; 1237-1239, 1982.
U.S. Pat. No. 5,378,723, issued Jan. 3, 1995 describes a series of thiaphysovenol carbamic acid derivatives which are indicated to exhibit high potency in the inhibition of AChE or BChE. The compounds of that invention were indicated, as in the case of U.S. Pat. No. 5,171,750 above, to be useful in treating disorders such glaucoma, Myasthenia Gravis, Alzheimer""s disease and poisoning with organo phosphates. As above, no specific indication is given as to which type of inhibitors would be used in which specified disorder.
Geula and Mesulam in a paper entitled xe2x80x9cCholinesterases and the Pathology of Alzheimer""s Diseasexe2x80x9d, Alzheimer""s Disease and Associated Disorders, Vol. 9, Suppl. 2, pp 23-28 (1995) make the following observations in summary: xe2x80x9cAlzheimer""s Disease (AD) is accompanied by a marked loss of acetylcholinesterase (AChE) activity associated with cortical cholinergic axons and cholinoceptive neurons. Simultaneous with this loss, cholinesterase (ChE) activity emerges in AD cortex in the form of AChE and BChE activity associated with plaques, tangles, and amyloid angiopathy. Our observations have shown that the ChE""s associated with the pathological lesions of AD (ADChEs) possess different enzymatic properties and quite possibly are of a different source as compared with the ChEs associated with normal neurons and axons. The ADChEs most likely have noncholinergic functions involved in the pathogenesis of AD.xe2x80x9d In a further section the authors at p.26 state: xe2x80x9cThese observations indicate that glia are a likely source of the ChE, and particularly the BChE, associated with the pathological lesions of AD. They also suggest that a high ratio of BChE to AChE positive glia may play a permissible or causative role in the neuropathology of this disease. It is possible that other pools of ChE exist with enzymatic properties similar or identical to those of AD ChEs. This possibility remains unexplored.xe2x80x9d
Workers in the art have indicated that BChE is found in significantly higher quantities in AD plaques than in plaques from age-matched non-demented brains. Moreover, BChE was found to alter the aggregation of beta amyloid peptide (Axcex2). It has been hypothesized that since AChE is inhibited by high concentrations of acetylcholine (ACh), while BChE remains unaffected, it may well be that BChE may play an important role in the in vivo regulation of synaptic concentrations of ACh in the brain of AD patients. BChE inhibitor, instilled into the brain has produced a significant increase in the level of extracellular Ach. (Giacobini, et al., Proc. Soc. Neurosci., 22; 203, 1996.)
It has also been found that in 3 AD specimens plaques, which were of the compact or neuritic type, were almost always associated with intense BChE activity. It was concluded that BChE activity appears at the intermediate stage of plaque formation and that it may therefore constitute one of the factors involved in the transformation of an initially benign Axcex2 deposit into a compact neuritic form associated with neural degeneration and dementia.
It has been unexpectedly discovered and thus forms the basis of the present invention that highly selective BChE inhibitors can be utilized by systemic administration to prevent or treat cognitive impairments associated with aging or Alzheimer""s disease in a host. Since BChE activity has previously been identified to reside primarily in peripheral organs, such as the pancreas, liver and serum or in the circulation and its inhibition was associated with side effects observed in first generation Alzheimer""s disease therapy (Liston et al., Proc. Soc. Neurosci., 20: 608, 1994.) Soreq and Zachnt, Human Cholinestrase and ?, Academic Press, New York, pp. 21-29, 1993) ref), the use of highly selective BChE inhibitors in the treatment or prevention of cognitive impairments associated with aging or Alzheimer""s disease was not suggested by the art. A further factor that pointed away from the possible use of highly selective BChE inhibitors in treating cognitive diseases of the brain and CNS was the expected distribution pattern of such agents. Data available in the art suggest that such compounds would be preferentially bound to peripheral organs where the major part of their substrate activity resides. It was, therefore, not expected that clinically useful concentrations of highly selective BChE inhibitors administered systematically to a patient would pass through the blood brain barrier and be available in the brain as (i) such compounds would be expected to be bound to systemic enzyme before reaching the brain, restricting its access, and (ii) most BChE inhibitors known in the art do not readily enter the brain. Indeed, until the present, inhibitors of BChE have been largly utilized as pesticides in agriculture. (Soreq and Zachut, Human Cholinesterases and Anticholinesterases, Academic Press, N.Y., pp. 21-29, 1993).
The term xe2x80x9chighly selectivexe2x80x9d as used herein is meant to include those BChE inhibitors whose ratio of IC (50) values against human plasma BChE compared to their IC (50) values against human erythrocyte AChE were greater than about 15 to 1.
The IC (50) values can be determined for such inhibitors using methods well known in the art. In such assay the pharmacological activity of each compound, as an IC (50), defined as the concentration, in nanomoles, required to inhibit 50% of the enzyme activity of AChE and BChE, is determined separately. For determination of IC (50) values, the enzyme activity of each concentration was expressed as a percent of that determined in the absence of each compound. This then was transformed into a logit format, where logit=In (% activity/[100-% activity]), and was plotted as a function of the log concentration of the compound. IC (50) values (i.e., logit=In (50/[100xe2x88x9250]=0) were determined only from correlation coefficients (r2) of less than xe2x88x920.985 and when more than 50% inhibition was achieved from duplicate samples. The selectivity ratio is then determined by comparing the IC (50) values obtained for each compound with AChE to that for BChE.